The present invention provides simple, effective and accurate cumulative measurement of gamma radiation. It can provide the value of gamma radiation to be used for calculation of radioactive gas concentrations. It can also provide a tissue equivalent dosage measurement of gamma radiation.
Measurements of radioactive gas are important for many purposes. Tritium concentrations in potentially exposed workers are measured, for example, with periodic urine specimens. Carbon-14 serves as a useful research tool for monitoring the progress of many chemical and biological reactions and interactions. For example, many microorganisms break down carbon-14 containing compounds in sugar to produce carbon-14 dioxide gas which can be collected and measured to determine various characteristics of the microorganisms. Both tritium and carbon-14 dioxide produce low energy radiation which cannot be easily measured by conventional radioactivity detectors. In measurement of radioactive gas concentration, the background level of high energy gamma radiation must be subtracted from the measured reading.
Radon (Rn-222) and thoron (Rn-220) are radioactive gases which are formed in the uranium and thorium decay series. They decay by alpha emissions with a half-life of 3.8 days and 55.4 seconds respectively. When they are formed near the surface of uranium containing materials such as soil or rock, they can diffuse out into the surrounding air where they and their daughter products can pose a radiological hazard to man under certain conditions. Each time a radon (Rn-222) atom decays, its daughter products, polonium-218 (Po-218), lead-214 (Pb-214), bismuth-214 (Bi-214), polonium-214 (Po-214), decay in sequence with half lives of 3.05 minutes, 26.8 minutes, 19.7 minutes, 0.16 milliseconds, respectively. The Po-218 and Po-214 are more hazardous than their radon gas parent because they emit very energetic alpha particles and they are particulates and can deposit in lungs when breathed. Once in the lungs, their high energy alpha emissions can damage tissue and may cause cancer. Thoron (Radon-228) which decays in a similar manner, is harmful to a lesser degree.
Radon and associated daughter products have long been known to be a causative agent for lung cancer when present in high concentrations usually found in uranium mines. More recently, concern has been expressed by many scientists over the high radon concentrations that have been measured in poorly ventilated homes all across the country. Hazardous radon concentrations often build up in homes, especially in "tightly" constructed energy-efficient homes and in those which have been retrofit sealed to conserve energy. The U.S. Environmental Protection Agency has estimated that 5,000-20,000 lung cancer deaths will occur annually in the United States as a consequence of this radon buildup in homes. The resulting concern over this hazard has given rise to a need for a low cost, passive instrument for measuring the concentrations of these natural radioactive gases. Similar health hazards are associated with breathing other radioactive gases such as tritium or carbon-14 dioxide in and around nuclear facilities.
Integrating-type monitors which measure the average concentrations of radon or other radioactive gases over a few days, weeks or months are especially useful because wide short-term fluctuations in concentration often occur due to perturbations in ventilation and atmospheric conditions. The present invention meets all of these needs. When used as a radon and/or thoron monitor, it is simple, small and rugged enough to be mailed to homeowners and back to the laboratory for readout. This eliminates the cost of technicians travelling to and from the homes to perform the monitoring. A miniature version can be worn to monitor workers for radon, thoron or tritium exposure. In another embodiment, it serves to monitor the very small quantities of tritium and carbon-14 dioxide emitted from biologically active cultures in certain measurements and experiments.
Several scientists have described various types of passive environmental radon monitors (PERMS) in recent years. However, only a few of them, e.g., A. C. George (Ref 1: A Passive Environmental Radon Monitor; Radon Workshop --Feb. 1977, HASL-325; 1977 p. 25) and C. CostaRiberio, et al. (Ref. 2: A Radon Detector Suitable for Personnel or Area Monitoring, Health Physics Vol, 17, 1969), utilized a thin metal plate maintained at a high negative voltage to collect the positively charged decay products of radon to gain increased measurement efficiency and accuracy. This enhanced accuracy is especially needed for the home monitoring application where radon concentrations are normally low. All of these workers took advantage of the fact that the radon daughter products are positively charged when formed. The alpha radiation emitted by the daughter products is measured either by thermoluminescent dosimeters or by alpha track detectors and the results are used to calculate the radon concentration.
The collection plates and the measuring detectors in these earlier devices were located inside filtered passive diffusion chambers which prevented the radon daughter products already present in outside air from reaching the detector. Only the parent radon gas can pass through the filter by passive diffusion to enter the measuring chamber. In these earlier devices, the radon gas which diffused into the chamber was indirectly monitored by measuring the radiation from the daughter products which are formed inside the chamber after they were collected on the surface of the collectors. They did not measure the parent radon gas directly. Radioactive gases such as C-14 dioxide and tritium do not form charged particulate daughter products. Therefore, the earlier inventions cited will not measure these gases. The present invention, however, will measure any radioactive gas because their radioactive emissions always generate ions in the chamber air. Further, the present invention uses an electret as a sensor which is different from the detectors used by earlier devices.
Kotrappa et al. (Ref. 3: Electret - A New Tool for Measuring Concentrations of Radon and Thoron in Air) also experimented with electrets for indirect monitoring of radon or thoron. They used negatively charged electret as a collector in the place of metal sheet maintained at a high negative voltage. They also measured alpha radiation of collected daughter products by scintillation detectors or by other known detectors.
In addition, they made an incidental measurement of charge on the polycarbonate sheet covered electret as a requirement of the experiments to ensure sufficient charge on the polycarbonate sheet to collect the daughter products. The difference in surface charge of the polycarbonate sheet before and after the experiment was not used for measurement of radon. However, they found a rather poor correlation between the difference in charge on the polycarbonate sheet (electret itself was not measured) to the cumulative gradon exposure and suggested further work.
There are two reasons why that earlier device gave a very poor correlation with radon exposure as follows: (1) The polycarbonate foil used by Kotrappa, et al. had a much higher electrical conductivity than the electret material which was fluorocarbon polymer. This conductivity caused the ions which collected on the polycarbonate foil to bleed off to ground much more readily than they do from the electret. (2) The adhesive tape and the air gap between the polycarbonate foil and the electret caused by the adhesive tape in the Kotrappa device also perturbed the ion collection and retention capability of the electret assembly substantially.
All of these factors contributed to the very poor correlation between radon exposure and surface voltage in the Kotrappa device and rendered it unsuitable for radon monitoring.
In another paper, Kotrappa et al. (Reference 4: Measurement of Potential Alpha Energy Concentration of Radon and Thoron Daughters Using an Electret Dosimeter, Rad. Prot. Dos. Vol. 5, No. 1 of p. 49-56 - 1983) measured the voltage difference on an electret to quantify the amount of alpha energy expended in air by radon and thoron daughter products which were captured on a filter. The system did not measure radon gas. The device developed by Kotrappa et al. in Ref. 4 also embodies a pump to transport the radon daughter products into the chamber.
A need exists for small compact rugged devices which are capable of accurately and dependably measuring radiological gases and integrating the measurements over known times.
H. B. Marvin (Reference 5: U.S. Pat. No. 2,695,363; Method and Apparatus for Measuring Ionizing Radiations, issued Nov. 23, 1954) used an electret to collect and store ions. The chamber in this earlier invention was sealed to prevent air entry so it measured only the gamma radiation which penetrated through the chamber wall.
This correlation between electret voltage and radon exposure using the present invention (i.e., with no adhesive tape or polycarbonate film), is excellent and it serves as a very accurate radon monitor. FIG. 1 shows this correlation using the present invention with a 225 ml cup-shaped chamber and 2.3 mm thick electret made of FEP Teflon.
The present invention also differs from devices which used real-time detectors (i.e., devices connected to real-time electronic readout systems). The present invention uses, instead, an electret type detector which records and integrates the positive or negative ions generated by the radon and radon daughter radiations without the need for connections to electronic devices during the radon exposure period. Real-time electronic equipment is too expensive and unwieldy for large scale home monitoring use.
Some monitoring devices are too large and heavy for home use because of the high voltage batteries or power supply utilized to maintain the charge on the detectors. Instead of batteries, the present invention uses a small precharged electret as described above. The electret ion-collection approach enables an accurate monitoring device which is small and rugged enough to be sent to homeowners through the mail. The use of a simple electret itself as a sensor rather than a solid state detector reduces the cost of the monitor and its readout equipment substantially.